In the field of this invention, techniques are known that use nanometer sharp tips for imaging and investigating the structure of materials down to the atomic scale. Such techniques include scanning tunnelling microscopy (STM) and atomic force microscopy (AFM), as disclosed in U.S. Pat. No. 4,343,993 and EP 0 223 918 B1.
Based on the developments of scanning tunnelling microscopy and atomic force microscopy, new storage concepts have been introduced over the past few years profiting from these technologies. Probes having a nanoscale tip have been used for modifying the topography and for scanning an appropriate storage medium. Data are written as sequences of symbols represented by topographical marks, such as indentation marks and non-indentation marks. The tips comprise apexes with a radius in the lower nanometer range and the indentation marks have a comparable diameter, for example, a diameter in the range of 20 to 30 nm or even smaller. Hence, these data storage concepts promise ultra-high storage area density.
In STM, a sharp tip is scanned in close proximity to a surface. A voltage applied between the tip and the surface gives rise to a tunnel current that depends on the tip-surface separation. From a data-storage point of view, such a technique may be used to image or sense topographic changes on a flat medium that represent stored information in logical “0”s and “1”s. In order to achieve reasonably stable current, the tip-sample separation must be maintained extremely small and reasonably constant. In STM, the surface to be scanned needs to be a conductive material.
In AFM, the sharp tip rests on one end of a soft spring cantilever. When the sharp tip is in close proximity to a surface, resultant forces therebetween can be sensed by the extent to which they cause bending of the spring cantilever.
A storage device for storing data based on the AFM principle is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on the mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. During operation, the probes scan an assigned field of the storage medium in parallel. That way, high data rates may be achieved. The storage medium comprises a polymer layer. The tips are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. For this purpose, the probes comprise cantilevers which carry the sharp tips on their end sections. Symbols are represented by indentation marks or non-indentation marks in the polymer layer. The cantilevers respond to these topographic changes in the surface while they are moved across the surface.
Indentation marks are formed on the polymer surface by thermo-mechanical recording. This is achieved by heating the tip of a respective probe via a write heater with a current or voltage pulse during the contact mode in a way that the polymer layer is softened locally where the tip touches the polymer layer. The result is a small indentation on the layer having a nanoscale diameter.
Reading is also accomplished by a thermo-mechanical concept. A read heater on the cantilever is supplied with an amount of electrical energy, which causes the heater to heat up to a temperature that is not high enough to soften the polymer layer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage medium, especially a substrate on the storage medium, changes when the probe is moving in an indentation as the heat transport is more efficient. As a consequence of this, the temperature of the cantilever decreases and, hence, its resistance changes. This change of resistance is then measured and determines the read-back signal. Reading and also writing the marks is accomplished by moving each probe relative to the storage medium along a line representing a track and moving to the next track when the end of the respective line has been reached. A thermo-mechanical probe with read and write capabilities is also referred to as a read transducer. The amplitude of a read-back signal is defined as the difference in magnitude between a read-back signal sample that is obtained when the tip of the probe is exactly at an indentation center, and a sample obtained when the tip of the probe is at an indentation-free area of the storage medium, while the probe moves along a track center line. If the probe is not exactly on track, the reference point for the measurement of the amplitude is defined as the point where the probe meets a straight line that crosses the indentation center in the cross-track direction. Typically, the amplitude decreases monotonically with the distance from a track center line and vanishes at half the track pitch. This is also disclosed in Eleftheriou, E., et al., “Millipede—a MEMS based Scanning-Probe Data-Storage System”, IEEE Transactions on Magnetics 39(2), March 2003, pp. 938-945.
EP-A-385161 discloses a storage device and a method for scanning a storage medium. The storage medium is designed for storing data in the form of marks and is scanned by an array of probes for mark detecting purposes in a scanning mode. The storage medium has fields with each field to be scanned by an associated one of the probes. At least one of the fields comprises marks representing operational data for operating the scanning mode. Scanning parameters are computed from the operational data and the scanning mode is adjusted according to the scanning parameters. The marks representing operational data may represent information for adjusting the position of the array of probes along a track. For that purpose, special marks are formed in the storage medium, preferably in respective fields of the storage medium, where such marks are preferably aligned along lines that are displaced in the cross-track direction relative to a track center line. By scanning the respective field comprising these marks, information on the actual position of the probes relative to the track center line can be derived and used for adjusting the position of the probe array in the cross-track direction. Other fields comprise marks forming periodic patterns along tracks. By scanning these fields, timing or clocking information may be obtained, which is used for adjusting the frequency of reading, writing or erasing pulses applied to the probes. These position or timing adjustments take effect for all of the fields and the respective allocated probes.
It is a challenge to provide a data storage device and a method for operating a data storage device, which enables the reliable retrieval of information with stringent requirements in respect to exact positioning of a read transducer, for a wide range of cross-track positions.